AP® Chemistry · Unit 9 · Topic 9.3
Gibbs Free Energy and Thermodynamic Favorability
Gibbs free energy, delta G = delta H minus T delta S, combines a reaction's heat change and its disorder change to predict whether the reaction happens on its own. A negative delta G means the process is thermodynamically favorable.
A reaction or process is thermodynamically favorable when delta G is negative. Delta G = delta H - T delta S packs two competing effects into one number. Delta H measures heat released or absorbed, and delta S measures how much more spread out the energy and matter become. Neither one alone decides favorability. Ice melting at room temperature absorbs heat, so delta H is positive, but the molecules go from a rigid lattice to a loose liquid, so delta S is strongly positive too. Multiply that entropy gain by room temperature and -T delta S becomes a large negative number, big enough to drag delta G below zero. That is why melting happens on its own above 0 C even though it costs energy.
Temperature is the dial that controls which term wins. At low T, -T delta S is small, so delta H dominates and exothermic processes tend to win out. At high T, -T delta S grows and can overpower a positive delta H, or cancel out a negative one. This is exactly why an exothermic reaction is not automatically favorable: if delta S is negative enough, -T delta S is positive, and at high enough temperature it can outweigh a negative delta H, flipping delta G positive. A delta G of exactly zero marks equilibrium, the system has no driving force left to move in either direction, and this condition is what lets you solve for a transition temperature.
This same balance of enthalpy and entropy reappears later as cell potential in electrochemistry, where delta G = -nFE links favorable reactions to positive voltages. Thinking in terms of delta H and delta S here sets up that connection directly.
Key terms
- Gibbs free energy (delta G)
- The quantity whose sign tells you whether a process is thermodynamically favorable under constant temperature and pressure.
- Thermodynamically favorable
- Describes a process with negative delta G, meaning it can proceed on its own without continuous outside energy input.
- Enthalpy (delta H)
- The heat absorbed or released by a reaction at constant pressure, negative for exothermic processes.
- Entropy (delta S)
- A measure of how spread out energy or matter becomes, positive when a system becomes more disordered.
- Equilibrium (delta G = 0)
- The state where a system has no net driving force to proceed forward or backward.
Watch it explained
- How it worksHow do enthalpy and entropy trade off inside delta G equals delta H minus T delta S?
- Worked exampleFind delta G for a reaction with delta H of minus 92 kJ and delta S of minus 199 J per K at 298 K.
- Common mistakeWhy isn't an exothermic reaction always thermodynamically favored?
- Going furtherWhat does a delta G of exactly zero say about the state of the system?
Check yourself
End of topic.